GPS Field Area Measure — Acres, Bigha, Guntha

About the GPS Field Area Measure — Acres, Bigha, Guntha

Measure a field, a farm or a plot by walking its boundary with your phone. The page records your track with the phone’s GPS through the browser, leaves out fixes that are too imprecise or jump too far, and closes the outline back to where you started. The area is worked out on the WGS 84 ellipsoid that GPS positions are given on and shown in acres, hectares, square metres, square feet, square yards (gaj), cents, gunthas and ares — and in kanal and marla (Haryana) or bigha, katha and chatak (West Bengal) with the sizes the Land Area Converter uses, or in a local bigha you define from your land record.

For a field with straight sides, Corners mode averages several fixes on each corner, which is steadier than one reading. No GPS signal? Tap the corners on a grid in metres or feet, paste coordinates, or open a GPX, KML, KMZ or GeoJSON file. Positions never leave your device. Without a pass the result is a free preview: the area in full, the first rows of each table and the sketch as a watermarked picture; a Pro pass unlocks every figure, the summary and the KML, GPX, CSV and PDF sketch files.

How the area is worked out

Each position (latitude φ, longitude λ on WGS 84: a = 6,378,137 m, f = 1/298.257223563) is turned into its authalic latitude β, the latitude on the sphere with the same surface area as the ellipsoid (radius R = 6,371,007.18 m). On a cylindrical equal-area map about the field’s middle, x = R cos β₀ (λ − λ₀) and y = R (sin β − sin β₀) ÷ cos β₀, areas are kept exactly, so the shoelace formula ½ |Σ (xᵢ yᵢ₊₁ − xᵢ₊₁ yᵢ)| gives the area of the boundary drawn with straight sides between the points.

Side lengths and bearings use the ellipsoid’s radii of curvature at the middle of each side. A walked track is straightened into sides wherever it runs within the GPS accuracy of a straight line, for the table of sides; the area always uses every point.

What the GPS accuracy means

Each fix comes with the device’s accuracy: the radius in metres that holds the true position with 95 % confidence (W3C Geolocation API). GPS-enabled smartphones are typically accurate to within 4.9 m under open sky, and worse near buildings, bridges and trees (GPS.gov).

The page leaves out fixes worse than your limit (12 m unless you change it), points closer than the step to the last one (2 m) and jumps faster than walking (8 m/s), and shows the median accuracy of the points kept. The band under the result, P·d + π·d² for a perimeter P and an accuracy d, is how much the area would grow if the whole outline sat d metres too far out. Errors partly cancel, so the real error is usually smaller — but on a small plot GPS is rough: measure it with a tape and the Plot Area Calculator.

Land units

The standard units are exact: 1 acre = 4,046.856 m² = 43,560 sq ft = 40 guntha = 100 cents, 1 hectare = 10,000 m² = 100 ares, 1 gaj (square yard) = 0.836 m². State units come only from official sources: kanal, marla and sarsai (and bigha and biswa before settlement) from the Haryana Revenue Department, and bigha, katha and chatak from the West Bengal Directorate of Registration & Stamp Revenue.

A bigha or a katha changes size from place to place, so for anywhere else open Your own local unit and enter its size from your land record or deed — for example bigha = 27,000 sq ft. It is used only on this page.

The files

• KML: the field as a polygon for Google Earth, QGIS and most map apps, with the corners as placemarks.
• GPX: the boundary as a closed track with the times and heights the phone gave, and the corners as waypoints.
• CSV: the area in every unit, the sides and every point with its accuracy, for a spreadsheet.
• PDF sketch: one page with the outline north up, the side lengths, a scale bar, the area table and the accuracy statement.

Limitations

• A GPS estimate, not a survey or a legal measurement. For buying, selling, dividing or registering land, have the boundary measured by a licensed surveyor.
• Keep this page on screen while you walk: browsers send no positions to a hidden page, so a stretch walked with the screen locked is missing (the page says so when you come back).
• Sides between recorded points are straight lines, and the area is the flat (horizontal) area of that outline — not the larger surface of a slope.
• On a plot of a few hundred square metres the GPS error is large compared with the plot; a tape measure is more exact there.
• At most 5,000 points per boundary, within 50 km of each other.

Privacy

Your positions, coordinates and files stay in this browser: nothing is uploaded. A walk in progress is kept on this device so a reload does not lose it, and fields you choose to keep stay here until you delete them.